Crouching Bird, Hidden Evolutionary Purpose?

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Birds can hold their wings high because of the strange way they
crouch, and now scientists say the origins of this folded posture
may provide insight into the evolution of their flight.

Birds stand and walk in an unusually crouched way, with the
femur, or thighbone, held nearly horizontally — unlike humans,
whose legs stand vertically. Birds’ crouched stance helps support
their balance and movement by ensuring
the center of gravity lies above the feet. Birds’ bipedal, or
two-legged, stance reflects their dinosaur heritage — dinosaurs
evolved a bipedal posture early in their evolution, about 235
million years ago.

The crouch seen in birds is quite different from the sprawling
posture seen in the closest living relatives of birds, the
crocodilians. Birds and crocodilians belong to a diverse
menagerie of creatures known as the archosaurs, which also
include all dinosaurs and the
extinct flying reptiles known as pterosaurs. The first
archosaurs, which appeared about 250 million years ago, resembled
modern crocodiles. Although the archosaurs, like modern
crocodiles, were four-legged animals with long, heavy tails, they
had longer limbs than crocodiles’ to make it easier for them to
live and move on land. [ Avian
Ancestors: Images of Flying Dinosaurs ]

Scientists disagreed on how birds evolved to crouch. Some
researchers contend this shift happened gradually in a group of
dinosaurs known as coelurosaurs, which included tyrannosaurs and
raptors. Others suggest this change occurred more suddenly,
beginning with the few immediate dinosaur ancestors of birds and
the origin of flight.

The dino crouch

To help resolve this debate, scientists explored the bird family
by analyzing 3D computer models of 17 archosaurs spanning about
250 million years of evolution. These included living birds, such
as chickens; what may be one of the first birds,
Archaeopteryx; the four-winged, feathered dinosaur
Microraptor; two-legged predators, such as Velociraptor
and Tyrannosaurus ;
and crocodiles, the closest living, albeit still distant
relatives of birds.

"We basically started from a simple digital 'shrink wrap' of the
whole skeleton," said researcher Vivian Allen, a biomechanist at
the Royal Veterinary College in Hatfield, England. "From this, we
expanded the 'shrink wrap' to match how much flesh we think
existed around the different parts of the skeleton. This was
based on both detailed reconstruction of the muscular anatomy of
each animal and on what we have measured from CT scans of their
living relatives."

Paleontologists had agreed for years that the crouch seen in
birds evolved as their tails became shorter, shifting the center
of gravity of certain dinosaurs progressively forward as those
creatures became more birdlike/This forced the legs to become
less vertical and more crouched to keep the center of gravity
balanced over the feet. [ Paleo-Art:
Stunning Illustrations of Dinosaurs ]

"Non-avian dinosaurs and archosaurs in general all have this very
large, muscular heavy tail, that obviously represents a
significant amount of mass on the back of the animal," Allen told
LiveScience. "So as you move along the evolutionary lineage of
birds, this is reduced in progressively more birdlike dinosaurs
and, eventually, is basically lost, or reduced to a small stump.
It seems very intuitive to suggest that this loss of the tail,
loss of this huge mass towards the back of the animal, would be
the main thing responsible for more birdlike dinosaurs and birds
themselves having more mass concentrated towards the front of the
animal."

Dinosaur forelimbs

Unexpectedly, the researchers found the evolution of this crouch
was more linked with their fronts than with their backs.

"Our results suggest enlargement of the forelimbs was more
important to the forward shift in center of mass than loss of the
tail," Allen said.

"The tail is the most obvious change, if you look at
dinosaur bodies," said researcher John Hutchinson, an
evolutionary biomechanist at the Royal Veterinary College in
Hatfield. "But as we analyzed and reanalyzed and punishingly
scrutinized our data, we gradually realized that everyone had
forgotten to check what influence the forelimbs had on balance
and posture, and that this influence was greater than that of the
tail or other parts of the body."

Since the forelimbs of the ancestors of birds eventually became
the wings of birds, these findings may provide insight into
the origin of bird flight.

"One of the interesting things that our work shows is that birds
could not have evolved these large forelimbs, these wings,
without also having to make significant changes to the anatomy
and function of their hindlimbs," Allen said. "Which makes total
sense, when you think about it — everything is attached to the
same body, so why wouldn't changing one thing affect the others?
But still, it was cool to find that, and to have some actual
numbers and stats to back it up."

As to when and how quickly the center of gravity changed position
in dinosaurs, the researchers found some merit to both sides of
the argument. "There were gradual changes early on in dinosaurs,
but we were amazed by how much the increase in forelimb size
began altering the center of mass just before when flight may
have first evolved in early birds and their closest relatives,"
Allen said.

Changes in body shape influence how animals balance, "and both
shape and balance are important for flight," Hutchinson told
LiveScience. For instance, if the center of gravity is close to
the wings, "then stable flight, such as gliding, is theoretically
possible," Hutchinson said.

Some of the crouching "seems to have started just a bit before
when flight seems to have evolved," Hutchinson said. "But it
didn't really take off until after flight evolved."

Dinosaur ancestors of birds may have evolved enlarged forelimbs
"for reasons other than powered flight, such as prey capture or
negotiating complex terrain," Hutchinson noted. [ In
Photos: Amazing Birds of Prey ]

Allen noted that the researchers analyzed only a few relatives of
birds. The researchers next plan to scan fossils of more
specimens to build computer models of their skeletons and "try
and get a clearer picture of what was going on in this really
interesting sequence of anatomical evolution," Allen said.

In addition, instead of looking at one or a few aspects of each
specimen at any given time — such as its mass, posture, skeletal
proportions and muscular anatomy — the scientists hope to analyze
every aspect of each specimen together simultaneously in
predictive computer simulations that animate their bodies. Such
models would help reveal how these
extinct animals stood and moved, and how that changed over
time.

"Developing techniques for looking at lots of complex data at the
same time — computer simulations of how the extinct animal worked
as a mechanism — that's definitely the way to go in the future,"
Allen said.

"It's just hard," she continued. "The maths are hard, and the
computing requirements are very high. But we're getting there.
What with very, very fast computers being increasingly dirt cheap
and the technical skills of researchers getting increasingly very
good, there are definitely some very exciting possibilities in
the near future of extinct animal mechanics studies."

Allen, Hutchinson and their colleagues, Karl Bates and Zhiheng
Li, detailed their findings online April 24 in the journal
Nature.